Reticles are used with photolithography tools (i.e., “steppers”) in the manufacture of semiconductor devices. Photolithography is one of the principle processes in the manufacture of semiconductor devices, and consists of patterning the surface of a semiconductor wafer in accordance with the active elements of the semiconductor devices to be produced. This is a layer-by-layer methodology, with each layer's manufacture including various deposition and etching processes, which are applied using a reticle. The reticle is stepped sequentially over the wafer and at each step the pattern of the reticle is transferred to the wafer, resulting in a complete layout on the wafer. To produce an operational microelectronic circuit, a reticle must be as defect-free as possible. Usually, the FAB engineers assume that the reticle has been tested in the reticle shop and is logically correct. Along the reticle life time in the production line, there is a need for the periodic inspection of the reticle's current condition to detect various defects therein that could potentially reduce the microelectronic circuit fabrication yields.
Various techniques for the reticle inspection have been developed. For example, it is known to detect defects such as the existence of foreign particles on the surface of a reticle, by utilizing the so-called “dark-field” inspection. According to this technique light that returns from the illuminated surface of the reticle at azimuth and elevation different from those where the most specular reflection occurs, is collected. This collected light is thus mainly formed by light components scattered from the foreign particles existing on the surface of the reticle.
U.S. Pat. No. 6,268,093, assigned to the assignee of the present application, discloses a method for reticle inspection using aerial imaging. According to this technique, defects such as line width errors in the printed image and surface defects can be detected. The line width errors are detected on the die area. Detection is performed by acquiring the image of the reticle under the same optical conditions as the exposure conditions (i.e. wavelength, numerical aperture, sigma, and illumination aperture type) and by comparing multiple dies to find errors in the line width. Surface defects are detected all over the reticle. The detection of surface defects is performed by acquiring transmission and dark-field reflection images of the reticle and using the combined information to detect particles, and other surface defects.
U.S. Pat. No. 6,466,314, assigned to the assignee of the present application, discloses a reticle inspection system and method. This technique consists of the following: A test reticle is generated comprising a plurality of test pattern-features thereon. Then, a wafer is manufactured using this test reticle, and a transfer of at least one of the plurality of pattern features from this reticle to the wafer is determined.
U.S. Pat. No. 6,614,520 discloses a method for inspecting a reticle for defects that occur over time. This technique utilizes generating and storing a “baseline” image of the reticle and then periodically generating a “current” image of the reticle and comparing the current and baseline images. The baseline image is taken at a time when the reticle is known to be acceptable. This may be when the reticle has been “qualified” by an optical test or when a die fabricated by reticle has passed an electrical test.